Typically, in electromagnetic flow meters for measuring the flow rate of a fluid that is electrically conductive, the flow rate of the fluid that flows within a pipe is measured by providing a magnetic excitation electric current that alternatingly switches polarities to a magnetic excitation coil that is disposed so that the direction of the magnetic field that is produced is perpendicular to the direction of flow of the fluid that is flowing within the pipe, to detect and differentially amplify, using a signal amplifying circuit, a signal electromotive force that is produced between a pair of detector electrodes that are disposed within the pipe perpendicular to the magnetic field produced by the magnetic excitation coil, and sampling and performing signal processing on the flow rate signal that is produced.
Such an electromagnetic flow meter, is provided with a fault detecting (empty detecting) function wherein, when the fluid within the pipe goes to empty so that the detecting electrodes of the detector no longer contact the fluid, or when a signal line in the detector is broken or disconnected, or the like, a fault state (an empty state) wherein a signal that has undergone differential amplification shows a value that is not normal, is detected, and an alarm is sent to a higher-level device. See, for example, Japanese Unexamined Patent Application Publication No. H8-021756.
FIG. 4 is a block diagram illustrating a configuration for a conventional electromagnetic flow meter.
In a signal amplifying circuit 52, flow rate signals that consist of signal electromotive forces that are detected by detecting electrodes TA and TB of a pipe Pex are read in through flow rate signal input terminals T11 and T12, and pass through buffer amplifiers U11 and U12 and coupling capacitors C11 and C12, to undergo differential amplification by an operational amplifier U13.
The output signal of U13 undergoes A/D conversion by an A/D converting circuit 55 after having the low-frequency component, such as power supply noise, removed by a high-pass filter (HPF) 54.
The controlling circuit (CPU) 56 performs a calculation process on the A/D-converted value to calculate a flow rate measured value, and notifies a higher-level device from the output interface circuit 57.
On the other hand, in a fault detecting circuit 53, the input electropotential V11 of one of the flow rate signals that are inputted into U13 is compared to a reference voltage Va by a comparator U21, and the comparison result V13 is outputted to the controlling circuit 56. Moreover, the input electropotential V12 of the other flow rate signals that are inputted into U13 is compared to a reference voltage Va by a comparator U22, and the comparison result V14 is outputted to the controlling circuit 56.
If in a normal state wherein the pipe Pex is filled with an adequate amount of fluid, then the detecting electrodes TA and TB and the ground electrode TC are connected electrically through the fluid. Because of this, the central electropotential between V11 and V12 is equal to the ground electropotential.
On the other hand, T11 and T12 in the signal amplifying circuit 52 are connected to power supply electropotentials +Vc and −Vc through resistive elements R11 and R12. Because of this, if the fluid within the pipe becomes empty, so that the detecting electrodes TA and TB of a detector 60 cease to contact the fluid, or if a signal line of the detector 60 becomes broken or detached, then V11 and V12 will go to +Vc and −Vc, respectively.
Consequently, setting the reference voltage Va for the fault detecting circuit 53 to a value that is higher than the negative amplitude peak value for the flow rate signal when operating properly and lower than +Vc, and setting the reference value −Va to a value that is lower than the positive-side amplitude peak value of the flow rate signal when operating properly and higher than −Vc makes it possible to detect a fault such as a state wherein there is no contact with the fluid or a state wherein a signal line is broken or disconnected.
In this way, in the signal amplifying circuit 52 of the conventional electromagnetic flow meter, the input impedance of the signal amplifying circuit 52 must be as high as possible so as to not produce attenuation in the flow rate signal, even if the fluid that is being measured has low electrical resistivity.
Because of this, FET input-type amplifiers, wherein the input impedance is high and the input bias current is low, are used as the buffer amplifiers U11 and U12.
However, while, in such a conventional technology, if it is possible to provide an adequate magnetic excitation current to the magnetic excitation coil Lex, to measure a normal electrically conductive fluid using a four-line electromagnetic flow meter wherein a relatively large signal electromotive force is produced, then there would be no problem even with the resistive elements R11 and R12 still connected, but if measuring, using the four-line method, a fluid of low electrical conductivity, or if the amount of magnetic excitation current that can be provided to the magnetic excitation coil Lex is limited, or with a two-line electromagnetic flow meter wherein the signal electromotive force that can be produced is extremely small, then the flow rate signal will be attenuated by the resistive elements R11 and R12, and thus there is a problem in that this will have an adverse effect on the signal-to-noise ratio.
Moreover, there is a problem with an increase in the contact resistance through adherence of insulating substances that are produced through electrochemical reactions between the electrodes of the detector and the fluid interface because of the supply of electric currents from the resistive elements R11 and R12 to the electrodes TA and TB.
Because of this, there is the need for countermeasures such as inserting switches in series with R11 and R12 and switching them OFF at the time of a flow rate measurement, to block the supply of the DC current to the TA and TB side, and switched ON to provide the electric current to the TA and TB side only when performing fault detection.
The present invention is to solve problems such as this, and an aspect thereof is to provide a fault detecting technology able to detect a fault state of a flow rate signal without supplying a DC electric current to the detecting electrodes through resistive elements.